The present invention relates to an improved spread spectrum communication method and communications system.
Referring first to FIG. 2, in a radio communications system utilizing spread spectrum modulation system, data to be transmitted applied at an input terminal 15 is multiplied by a spread code generated by a spread code generator 16 in a spread modulator 17. Here, the multiplication of digital data means a logical exclusive OR operation.
Specifically, the value (1 or 0) of binary data to be transmitted is left as it is when the value of the spread code is zero, while the polarity of the data to be transmitted is inverted when the value of the spread code is one.
Explaining in detail with reference to FIG. 3, where data to be transmitted is represented by (A) and a spread code by (B), the data to be transmitted (A) is spread modulated on the spread code (B) to generate a signal as illustrated in FIG. 3(C). As can be seen from FIGS. 3(A)-(C), the modulated signal (C) has the polarity of data reverse to the polarity of the spread code (B) during a period in which the data to be transmitted (A) is at one level.
Here, the spread code may be random data having a data rate which is an integer multiple of that of the data to be transmitted, and an M-series code, for example, may be used therefor. The M-series code has a finite recursive period (for example, approximately 1,000 bits), and may be regarded as a pseudo random code.
As a result of the multiplication of the data to be transmitted (A) by the spread code (B), the data to be transmitted is converted into a random code having the data rate equal to that of the spread code, as illustrated in FIG. 3(C).
Assuming that the data rate of the data to be transmitted is f.sub.T and the data rate of the spread code is f.sub.D, the latter is divided by the former to derive g (f.sub.D /f.sub.T =g) which is called the "spread gain".
A bandwidth occupied by the data to be transmitted depends on the contents of the data (amount of information). When the data is random (including many high frequency components), it has a frequency bandwidth substantially corresponding to f.sub.T.
A bandwidth occupied by the data to be transmitted multiplied by the spread code does not depend on the contents of the data to be transmitted, in which case the resulting data has a frequency bandwidth substantially corresponding to f.sub.D.
Referring back to FIG. 2, an output of the spread modulator 17 is modulated, for example, in accordance with the BPSK (binary phase shift keying) scheme in a modulator 18, and then converted into a high frequency signal in a frequency convertor 19. In the radio transmission in accordance with the spread spectrum modulation scheme, a 2.4 GHz bandwidth in an ISM (industrial scientific and medical equipment) band is used as a frequency band for such high frequency signals. The high frequency signal is transmitted through a transmission antenna 20.
The high frequency signal transmitted from the transmission antenna 20 is received by a reception antenna 21 at a receiving station, and converted into an intermediate frequency signal in a frequency convertor 22. Then, the intermediate frequency signal is demodulated to random data having the data rate f.sub.D in a BPSK demodulator 23.
Explaining in greater detail with reference again to FIG. 2, a spread code generator 24 generates the same spread code as that generated on the transmission side (called the "inverse spread code). The inverse spread code is multiplied by the demodulated random data in the inverse spread demodulator 25. As a result of the multiplication in the inverse spread demodulator 25, if the inverse spread code has the value of zero, the value of the demodulated data is left as it is. Conversely, if the inverse spread code has the value of one, the polarity of the demodulated data is inverted.
In other words, when received data, identical to the spread modulated data to be transmitted (C) as illustrated in FIG. 3, is multiplied by the inverse spread code identical to the spread code (B), the received data identical to the data to be transmitted (A) is derived.
Thus, if the recursive period and the phase of the spread code on the transmission side are maintained identical to those of the inverse spread code on the reception side, received data equal to transmitted data is delivered at an output terminal 26.
The spread spectrum scheme features that an originally imparted bandwidth f.sub.T of data is expanded to a wider bandwidth f.sub.D for transmission, so that the S/N (signal to noise) ratio of a received signal is increased by a factor of g equal to the spread gain, thus enabling signals to be transmitted even on a transmission path exhibiting a low S/N ratio.
In the spread spectrum communications scheme, a C/N (carrier to noise) ratio is degraded in a receiver under communication when another spread spectrum transmitter approaches thereto. More specifically, assuming that the approaching spread spectrum transmitter has a carrier (a portion inputted to the receiver under communication) C1, noise N introducing into the receiver is increased to N+C.sub.1, whereby the C/N ratio is degraded to C/(N+C.sub.1). In other words, other nearby transmitters may interfere with a receiver under communication.
U.S. Pat. No. 5,056,109 discloses that the transmission power of a mobile station is adjusted in accordance with a communication distance from the mobile station to others in a cellular mobile telephone system to prevent the carrier of the mobile station from interfering with other nearby mobile stations, as mentioned above.
The transmission power may be controlled by a variety of schemes, such as open loop control, closed loop control, and so on. However, the addition of any control scheme would make the entire system more complicated.